1. Field of the Invention
The present invention relates to a magnetic thin film and a magnetic device using the same. More specifically, the present invention relates to a soft magnetic thin film that is useful for a magnetic recording head, a magnetic reproducing head, a magnetic sensor including a magnetic impedance sensor, a magnetic circuit component such as a magnetic coil and an inductor, or magnetic inductance heating equipment such as an IH rice cooker and an IH hot plate, and a magnetic device such as a magnetic head, a magnetic sensor, a magnetic circuit component, and magnetic inductance heating equipment using the soft magnetic thin film.
2. Description of the Prior Art
A magnetic material having both an excellent magnetic property and a high saturation magnetic flux density has been demanded in the field of a magnetic device using a soft magnetic material. To be specific, an improvement in the writing ability of a magnetic head involved in the improvement in magnetic recording density, an improvement in the rate of change of magnetic impedance of a magnetic impedance sensor, and an improvement in the efficiency of the conversion from electromagnetism to heat of magnetic inductance heating equipment are desired. In order to seek a material satisfying those demands, transition metal (Fe, Co)--IIIa to Va or IIIb to Vb based-materials have been recently studied in a wide range (e.g., Hasegawa: Journal of Japan Applied Magnetism, 14, 319-322 (1990), NAGO IEEE, Trans, magn., Vol. 28, No.5 (1992)). These many studies have established that it is important that a material exhibiting a soft magnetic property among the aforementioned compositions has an amorphous phase or a microcrystal phase close to the amorphous phase immediately after the formation of a film, then grains are growth by a heat treatment or the like, and the material finally has a granular structure. Furthermore, in regard to the crystal size of the granular particles, many researchers including Herzer (IEEE, Trans. magn., MAG-26, 1397 (1990), Journal of Japan Applied Magnetism Vol. 20. No.6 (1996)) have confirmed the followings. An excellent soft magnetic property can be produced, only when an average crystal size of magnetic crystal grains is sufficiently smaller than a distance of exchange coupling or sufficiently larger than that. According to many reports, the mechanism of this production is as follows. In a region with large crystal grains, domain wall motion due to defects or reduction in a grain boundary density or easiness of magnetization rotation produces the soft magnetic property. On the other hand, in a region with small crystal grains, the soft magnetic property is realized in the following manner: each microcrystal grain significantly interacts with adjacent microcrystal grains for three-dimensional exchange so as to offset each crystal magnetic anisotropy, and thus reducing an apparent crystal magnetic anisotropy.
A microcrystal material in which precipitated or grown microcrystal grains are substantially composed of a magnetic metal (e.g., Fe, FeCo), especially a material having a high saturation magnetic flux density of 1.2 T or more, poses a problem of corrosion resistance. Therefore, an improvement in corrosion resistance is attempted by dissolving an element such as Al that forms a passive state in .alpha.-Fe. However, an anti-corrosion element forming a passive state such as Al basically preferentially reacts with a light element such as oxygen, nitrogen, carbon, or boron used for producing an amorphous state or making crystal grains smaller, because it has a low free energy for the formation of an oxide and a nitride. Thus, the anti-corrosion element is unlikely to remain in a solid solution with .alpha.-Fe microcrystals. In the case that an amount sufficient to provide corrosion resistance is added to the .alpha.-Fe microcrystals, the saturation magnetic flux density is lowered significantly.
On the other hand, when these magnetic materials are used for a magnetic head, the material is subjected to a heat treatment in a process for fusing with a glass that is necessary for producing a magnetic head. The melting point of the glass, the coefficients of thermal expansion of the substrate, the glass and the magnetic film, the optimum microcrystal precipitation temperature of the magnetic material and the matching of them influence the characteristics of the magnetic head. The temperature for the heat treatment to produce a head is preferably 500.degree. C. or more in view of the reliability of the glass and the optimum temperature for the heat treatment for the magnetic material.
When the magnetic head is a metal-in gap head (MIG head) in which a magnetic thin film is formed, for example on ferrite, when the temperature in the heat treatment is excessively high, a reaction proceeds at the interface between the ferrite and the magnetic film, so that a magnetism-degraded layer produced at the interface between the magnetic film and the ferrite becomes thicker, and thus pseudo-gap noise becomes larger. In the case of a LAM head in which a magnetic thin film and an insulating film are laminated on a non-magnetic substrate, the magnetic film has a different coefficient of thermal expansion from that of the substrate. Therefore, thermal stress between the magnetic film and the substrate becomes larger as the temperature in the heat treatment is higher. Thus, the soft magnetic property of the film is degraded due to an increase of anisotropic energy caused by an inverse magnetostriction effect. Therefore, it is desired that the optimum temperature in the heat treatment for the magnetic material is about 550.degree. C. or less.
However, as described above, the microcrystal material comprising a sufficient amount of an anti-corrosion element in the solid solution with metal microcrystals is required to be subjected to a heat treatment at a temperature in the vicinity of 600 and 700.degree. C. or more in order to stabilize the crystal structure and allow a sufficiently small magnetostriction constant.
Furthermore, these microcrystal magnetic thin films inherently have a number of interfaces present between magnetic particles per unit volume. Therefore, magnetic crystal grains are grown significantly during a heat treatment by using the interface energy as a driving force. This results in a narrow range of the optimum temperature in the heat treatment exhibiting a satisfactory soft magnetic property, heterogeneous properties and a limited range of the temperature for use.
On the other hand, peeling of a film from a substrate due to internal stress and a fine crack on a substrate are problems common to many thin film materials. For example, the internal stress of a film that is formed on a substrate by sputtering generally includes compression stress or tensile stress. When the adhesive strength between a substrate and a film or the breaking strength of a substrate material is weak, the problem of peeling of the film occurs, depending on the shape or the surface state of the substrate.